U.S. patent number 5,300,331 [Application Number 08/019,824] was granted by the patent office on 1994-04-05 for method and apparatus for uv curing thick pigmented coatings.
This patent grant is currently assigned to Fusion Systems Corporation. Invention is credited to William R. Schaeffer.
United States Patent |
5,300,331 |
Schaeffer |
April 5, 1994 |
Method and apparatus for UV curing thick pigmented coatings
Abstract
A method and apparatus for the ultraviolet curing of pigmented
coatings having a thickness from about 1 mm to about 10 mm at a
high curing rate and a relatively low energy consumption per unit
area of coating cured. A coated substrate is irradiated by a first
high UV source having a spectral energy distribution of the
discharge which is high in the range from about 350 nm to about 450
nm and by a second UV source having a spectral energy distribution
of the discharge which is high in discrete regions through the
range of about 200 nm to about 450 nm. Each UV source is powered by
at least about 300 watts per inch of width of the coating.
Inventors: |
Schaeffer; William R. (Mt.
Airy, MD) |
Assignee: |
Fusion Systems Corporation
(Rockville, MD)
|
Family
ID: |
25075957 |
Appl.
No.: |
08/019,824 |
Filed: |
February 19, 1993 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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766279 |
Sep 27, 1991 |
|
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Current U.S.
Class: |
427/493; 427/393;
427/397; 427/492; 427/514 |
Current CPC
Class: |
B05D
3/067 (20130101); B29C 35/08 (20130101); B29C
2791/001 (20130101); B29C 2035/0827 (20130101) |
Current International
Class: |
B05D
3/06 (20060101); B29C 35/08 (20060101); B05D
003/06 (); B05D 003/02 () |
Field of
Search: |
;427/514,508,180,393,397,492,493 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"UV-Curable Unsaturated Polyester Systems for the Industrial
Finishing of Furniture" Dr. Riberi, Seminar of Radtech Intl. North
America, Atlanta, Ga., Aug. 23, 1990..
|
Primary Examiner: Padgett; Marianne
Attorney, Agent or Firm: Pollock, Vande Sande &
Priddy
Parent Case Text
This application is a continuation of application Ser. No.
07/766,279 filed Sep. 27, 1991, now abandoned.
Claims
We claim:
1. A process of curing an ultraviolet radiation curable pigmented
coating of a coated substrate with a coated surface and an opposing
surface, said coating having a thickness of at least about 1 mil,
said process comprising the steps of:
linearly focusing radiation on said coated substrate from a first
source of ultraviolet energy comprising a linear lamp having a
first spectral energy distribution wherein at least about 50% of
its spectral energy is in the range from about 300 to about 450
nanometers, and which is powered at an energy level of at least
about 300 watts per inch of lamp length;
linearly focusing radiation on the resulting irradiated coated
substrate from a second source of ultraviolet energy comprising a
linear lamp having a second spectral energy distribution which
differs from said first spectral energy distribution and has its
spectral energy distributed in discrete regions throughout the
range of about 200 to about 450 nanometers, said second source of
ultraviolet energy being provided with an input power of at least
about 300 watts per inch of lamp length; and,
conveying the coated substrate past said first and second sources
of ultraviolet energy during the steps of irradiation.
2. A process according to claim 1 wherein said first and said
second ultraviolet energy sources are powered at at least about 500
watts per inch of width of the coating.
3. A process according to claim 1 wherein said first and said
second ultraviolet energy sources are powered at at least about 600
watts per inch of width of the coating.
4. A process according to claim 1 wherein said pigment coating is a
light-colored coating, said first and second sources of ultraviolet
radiation are each powered at at least about 400 watts per inch of
coating width, and at least about 50% of the spectral energy from
said first source of ultraviolet energy is in the range of 400 to
about 450 nanometers.
5. A process according to claim 4 wherein said first and said
second ultraviolet energy sources are powered at at least about 500
watts per inch of width of said coating.
6. A process according to claim 1 wherein said thick pigment coated
is a dark coating, and at least about 60% of the spectral energy
from said first source of ultraviolet energy is in the range of
about 350 to about 450 nanometers.
7. A process according to claim 6 wherein said second ultraviolet
energy source is powered at at least about 500 watts per inch of
width of the coating.
8. A process according to claim 6 wherein said second ultraviolet
energy source is powered at at least about 600 watts per inch of
width of said coating.
9. A process according to any one of claims 1, 4, or 6 wherein said
coating is selected from a group consists of acrylate and urethane
based coatings.
10. A process according to any one of claims 1, 4, or 6 wherein the
total energy applied to said first and second sources of energy is
less than about 50 joules per square centimeter of coating
cured.
11. A process according to any one of claims 1, 4, or 6 wherein
said substrate is conveyed past said first and second ultraviolet
energy sources at a rate of no less than about 5 meters per
minute.
12. A method according to any one of claims 1, 4, or 6 wherein said
substrate articles are conveyed past said first and said second
ultraviolet energy sources at a rate of at least about 9 meters per
minute.
13. A process according to claim 1 wherein said substrate is
irradiated with ultraviolet from more than one first ultraviolet
radiation source.
14. A process according to claim 1 wherein said substrate is
irradiated with ultraviolet from more than one second ultraviolet
radiation source.
15. A process according to any one of claims 1, 4 or 6 wherein said
coating is about 2 to about 10 mils thick.
16. A process according to any one of claims 1, 4 or 6 wherein said
coating contains pigment in the amount of about 10 to about 40
percent by weight.
17. A process according to any one of claims 1 or 6 wherein said
coating contains carbon black.
18. A process according to claim 4 wherein said coating contains
titanium dioxide.
19. A process according to claim 1 wherein:
said coated substrate includes a coated upper surface, a lower
surface and a coated edge surface; and,
said radiation is directed towards said upper surface and focussed
at or near said lower surface.
Description
FIELD OF THE INVENTION
The present invention pertains to processes and apparatus for
ultraviolet curing thick, heavily pigmented coatings.
BACKGROUND OF THE INVENTION
Apparatus and processes currently used for curing thick pigmented
coatings on furniture pieces are typically designed to cure the
coating at a linear rate of about 8 meters/minute. In a first step
2-3 mils (100-125 microns) of a UV curable composition may be
applied by spraying, roll coating or curtain coating for example,
on a substrate such as a medium density fiber board (MDF). After
coating, the MDF is introduced into a flash-off tunnel to remove
residual volatile solvents the concentration of which typically
ranges from about 30% to about 40% by weight. The residence time is
usually 3 minutes at approximately 50.degree. C. in circulating
air. To maintain a 3 minute dwell in the flash-off tunnel at 8
meters/minute a tunnel 24 meters long is required.
After the residual solvent has been removed from the coating, the
substrate is introduced into a tunnel for initial curing. This
tunnel may contain, for example, TL03 lamps having these trate
designations and peaked at 420 nm alternating with TL05 lamps
operating in 360 nm range of the UV spectrum. Such lamps,
manufactured by N. V. Phillips, are low pressure lamps each of
which produces 120 watts of energy and is 48 inches long. This
initial curing step also requires a 3 minute dwell time, and to
maintain the rated line speed of 8 meter/minute, a 24 meter long
tunnel is required. These low pressure lamps may be positioned, for
example, on 12.7 cm centers along the entire length of the 24 meter
tunnel. This length requires 192 low pressure lamps to carry out
the initial curing phase of the process.
At this point the coating is soft and undercured at the surface. To
impart the hard, scratch-resistant properties required of this
coating, a final cure phase is required. The final cure is
accomplished using medium pressure UV lamps operating at 80 w/cm.
Three rows of lamps can achieve a line speed of approximately 2.5
meters per minute. For example, the first row of lamps may have an
output peaked at 420 nm while the next two rows of lamps may have a
365 nm peak. In order to reach a desired production rate of 8
meters/minute, a total of 9 rows of 80 w/cm lamps is required, the
first 3 rows having lamps producing the 420 nm energy, and the
following 6 rows producing the 365 nm output. The total lamp input
power per unit area is about 65 joules per square centimeter of
cured coating. About 1/5 of this energy comes from the TL lamp. The
light shielding and conveyor system to accommodate 9 rows of lamps
is approximately 4 meters long.
The combined length of the flash-off tunnel, the initial cure zone
and the final cure station is approximately 52 meters.
In some cases particularly when the pigment coating is white, in
order to obtain the desired level of opacity, multiple thin layers
must be applied using the above process.
When the low intensity TL lamps are employed in the initial cure
zone of the process, the coating which is applied to the edge of
the substrate may be tacky to the touch or in some cases may even
be wet and uncured. In many cases, to obtain a complete cure on the
vertical edges of the MDF board, a secondary cure mechanism is
employed.
Typically cobalt peroxide is added to the coating composition and
cure on the vertical edges is then completed in a matter of hours;
however, the addition of cobalt peroxide to the coating has some
decided disadvantages: (a) the pot life of the composition is
greatly reduced, making it imperative that the coating equipment be
thoroughly purged of coating material by the end of the work shift;
and (b) the cobalt peroxide may cause premature gelation of the
coating in the application equipment making laborious and
time-consuming cleaning necessary, and causing the loss of valuable
production time.
Additional disadvantages of the above described equipment and
process are: (a) the high floor space requirement, which precludes
its installation in small job shops, (b) high maintenance costs due
to the large number of lamps which are needed, (c) the large number
of parts in process at any given time, and d) in going through the
curing line the article is heated sufficiently so that it must be
cooled when it comes off the line so that substrates do not stick
together when stacked.
SUMMARY OF THE INVENTION
It is one object of the present invention to provide an improved
method and apparatus for curing, thick pigmented layers coated on a
substrate.
It is another object of the invention to eliminate the need for a
low intensity light tunnel for curing thick pigmented coatings.
It is still another object of the invention to eliminate the need
to use, and the above mentioned disadvantages associated with, a
peroxide catalyst such as cobalt peroxide, in thick pigmented
coatings.
It is yet another object of the invention to reduce the floor space
requirement of a thick pigmented coating curing line.
It is yet another object of the invention to provide a process and
apparatus for; completely curing coatings on thick substrate
articles including coatings borne on vertical or substantially
vertical portions thereof.
It is yet another object of the invention to eliminate the need to
cool articles bearing thick pigmented coatings after they are taken
off the curing line and before they are stacked.
It is a further object of the invention to provide a process which
reduces the energy required for curing thick pigmented
coatings.
According to a first aspect of the invention, thick, pigmented
coatings are subjected to ultraviolet radiation from a high
intensity discharge lamp such as a medium pressure doped mercury
lamp, which radiation is peaked at about 385 nanometers, at least
about 50% of which is in the range of about 350 to 450 nanometers.
The coatings are then treated with ultraviolet radiation from a
high intensity non-doped discharge lamp, the spectral energy
distribution of which is relatively high in discrete regions
throughout the range of about 200 to about 450 nanometers. Both
lamps are powered at at least 300 watts per inch of the width of
the substrate. The second lamp is preferably powered at at least
about 500 watts per inch and more preferably at about 600 or more
watts per inch.
In particular, according to one aspect of the present invention
relatively dark, for instance black, thick pigmented coatings are
cured. Such coatings are first preferably subjected to ultraviolet
radiation from a high intensity discharge and more preferably a
medium pressure doped mercury lamp emitting radiation which
normally peaks at about 385 nanometers, at least about 50%,
preferably about 60% of which is in the range of about 350 to about
450 nanometers, and then treated with ultraviolet radiation from a
higher intensity discharge lamp such as a medium pressure non-doped
mercury lamp.
According to a further aspect of the invention relatively light
colored, e.g. white, thick pigmented coatings are first subjected
to ultraviolet radiation from a high intensity discharge medium
pressure doped mercury lamp, which radiation is peaked at about 410
nanometers, at least about 40%, preferably about 50% of which is
emitted in the range of about 400 to 450 nanometers, and second
treated with ultraviolet radiation from a high intensity discharge,
medium pressure, non-doped mercury lamp. Both lamps are powered at
at least 400 watts per inch of width of substrate to be cured and
are more preferably powered at about 600 or more watts per inch of
substrate.
According to a still further aspect of the invention the curing
lamp which comprises an elliptical reflector and an ultraviolet
emitting lamp located at a first focus, is located so that the
second focus of the ellipse corresponds to the bottom surface of a
substrate article to be cured. Most of the coating to be cured is
borne on the top surface. The position of the lamp results in
substantially complete curing of a coating borne on vertical or
substantially vertical side walls of the substrate article.
The energy input to the lamps per square centimeter of coating
cured in carrying out the present invention is less than about 50
joules, is preferably less than about 40 joules and is most
preferably less than about 35 joules.
The coating can be cured by conveying coated substrate articles
past the ultraviolet sources at a rate of at least about 9 meters
per minute.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a graph of the spectral energy distribution of a high
intensity discharge, medium pressure, non-doped mercury lamp used
according to the invention.
FIG. 2 is a graph of the spectral energy distribution of a high
intensity discharge, medium pressure, doped mercury lamp, the
output of which is peaked at 385 nanometers and is substantial in
the range of 350-450 nanometers, which is used according to the
invention.
FIG. 3 is a graph of the spectral energy distribution of a high
intensity discharge, medium pressure doped mercury lamp, the output
of which is peaked at 410 nanometers and is substantial in the
range of 400 nanometers to 450 nanometers.
FIG. 4 is schematic perspective view of an ultraviolet curing
station.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As used herein, ultraviolet is defined as radiation within the
wavelength between about 200 and 450 nanometers.
The substrate to which the process is applied may be formed from a
variety of materials such as, for example, masonite, wood and MDF.
According to one process, masonite is coated with a white thick
pigmented coating to form a dry erase white board. The coating
method and apparatus might also be applied to wood boards. However,
the use which is expected to be most prevalent, and which is
described in the background section, is the coating of MDF. MDF
having a thick pigmented coating, particularly black or white, is
commonly used in furniture, e.g., shelves, tables, etc. MDF for use
in furniture is usually about one inch thick and is coated on
vertical as well as a horizontal surface edges.
The coating treated in accordance with the present invention is a
thick pigmented coating. The term "thick" as used herein refers to
a coating which is at least about 1 mil thick and preferably about
2 mils to about 10 mils thick. In the case of the black coating the
pigment can be provided in the form of carbon black. In the case of
the white coating the pigment may be provided in the form of
titanium oxide. The pigment can be usually provided in an amount of
about 10% to about 40% by weight of the solids content of the
finished coating. The higher amounts of pigments, i.e., those in
the range of about 40%, are provided in high quality coatings. By
way of a non-limitive example the coating may be of the acrylate or
urethane type, and other photocurable coatings can be used when
desired. A preferred black coating is High Gloss Black UV Topcoat
#EA090W, and a preferred white coating is High Gloss White UV
Topcoat #EA074W. Both of these coatings are manufactured by Crown
Metro, Inc., Greenville, S.C.
Examples of known methods for applying coatings include roll
coating, spray coating and curtain coating. Roll coating has the
advantage that the coating equipment is relatively inexpensive, but
it also has the disadvantage that it presents problems in laying
down a thick coating. Spray coating can provide a thick level
coating yet a certain amount of spray misses the substrate and is
wasted. Curtain coating provides a smooth coating the thickness of
which can easily be controlled and it does not waste coating
material. The method of applying the coating does not affect the
use of the process and apparatus for curing the coating according
to the present invention.
FIG. 1 is a graph of the spectral energy distribution of a high
intensity discharge, medium pressure non-doped mercury lamps used
according to the invention. The spectrum is characteristic of
primary radiating component, i.e., mercury. Such lamps generally
comprise mercury and up to a few hundred Torr of argon which is
provided for starting purpose. The filling of such lamps is known.
The lamp may be an arc lamp or a microwave electrodeless lamp. Both
are known types of discharge lamps. As is apparent from FIG. 1, the
non-doped mercury lamp emits radiation in selected bands throughout
the ultraviolet spectrum from about 200 to about 450
nanometers.
FIG. 2 is a graph of the spectral energy distribution of a high
intensity discharge, medium pressure doped mercury discharge lamp,
the output of which peaks at about 385 nanometers and is primarily
in the range of 350-450 nanometers, which range is effective in
curing thick UV-curable coatings according to the invention. The
spectral energy distribution shown is best obtained by doping a
mercury lamp bulb with a small amount of iron. Fusion Systems Corp,
the assignee of the instant invention, manufactures high intensity
discharge, medium pressure discharge, microwave electrodeless
lamps, and offers bulbs identified as the V-Bulb for those lamps
which produce a spectrum as shown in FIG. 2.
FIG. 3 is a graph of the spectral energy distribution of a high
intensity discharge, medium pressure doped mercury lamp, the output
of which peaks at 410 nanometers and is most intense in the range
of 400 nanometers to 450 nanometers. The spectral energy
distribution shown is best obtained by doping a mercury lamp with a
small amount of gallium. Fusion Systems also offers a bulb
identified as the D-Bulb for a lamp which produces a spectrum as
shown in FIG. 3.
The spectrums shown in FIGS. 2 and 3 may be produced by arc lamps
which are doped with iron and gallium, respectively. However, at
the current state of the art arc lamps, using dopants, have power
which is too limited because at high power the period for which the
spectrums characteristic of the dopant persists is short. Since the
method according to this invention requires high power, microwave
electrodeless lamps are preferred for the lamps from which the
radiation according to FIGS. 2 and 3 is to be obtained.
In FIG. 4, which shows an ultraviolet curing station, a first lamp
1 is arranged in relation to a conveyor 2 so that a line at which
radiation from the lamp 1 is focused is across at least a portion
of the width of the conveyor 2 through which coated substrate
article(s) 3, such as an MDF pass. The lamp optics comprise a
specular elliptical cylindrical reflector 4 having a linear bulb 5
disposed at a first focus F1 of the elliptical cylindrical
reflector 4. The elliptical reflector 4 serves to collect and
concentrate the ultraviolet emitted from the linear bulb 5 on a
line at the second focus F2. The lamp 1 is positioned so that the
second focus line F2 is located at the lower surface 6 of the MDF
substrate 3 rather than the top surface 7 which bears the
coating.
The ultraviolet at the second focus is increased to at least about
10 times compared to its intensity without the reflector, i.e., the
isotropic radiation from the source. Without the reflector, the
intensity is higher one inch above the line of the focus than it is
at the line of the focus because it is closer to the source.
However, with the reflector, the intensity one inch above the focus
is less than at the focus. For example, the ultraviolet intensity
one inch above the focus is about 5 times what it would be without
the reflector.
The prior art practice has been to position the lamp relative to
the substrate so that the second focus line is at the surface
bearing the coating.
Most of the coating on an MDF is borne on the top surface and a
small percentage is borne on the vertical edges. It has been
discovered by the inventor that positioning the lamp so that the
second focus is at the bottom of the MDF results in good curing of
the coating on the top surface and the vertical edges. Maximum
intensity available from the lamp is not obtained at the top
surface and it is uncertain that it is obtained at the bottom of
the vertical edge even though that is at the second focus line,
because of the orientation of the edge. Nonetheless the
advantageous results were obtained.
A second lamp which is the same as described above is located
beyond the first lamp in the direction of the conveyor
movement.
Substrate articles such as a MDF pass on the conveyor under the
illumination of the two lamps. The height of the lamps above the
conveyor is adjusted so that the second focus of the reflector is
at the bottom of the substrate articles rather than the top which
bears most of the coating. Coating borne on the top and vertical
edges of the substrate article is cured.
According to a preferred aspect of the invention directed to curing
relatively dark coatings the first lamp emits radiation which is of
the type described in connection with FIG. 2 and is supplied with
power in the amount of at least about 300 and preferably at least
about 400 watts per inch of its length. Further, according to this
aspect of the present invention, the second lamp is of the type
described in connection with FIG. 1 and is supplied with power in
the amount of at least about 500 and preferably at least about 600
watts per inch of its length.
If the lamps are operated at the preferred levels the conveyor may
be run at about 9 meters/min and good curing is obtained. Thus
effective cure is obtained at about 262/3 joules of lamp input
energy per square centimeter of substrate.
According to another preferred aspect of the invention directed to
curing relatively light colored coatings both lamp bulbs are
supplied with power of at least about 500 and preferably at least
about 600 watts per inch of their length. The first lamp is of the
type described in connection with FIG. 3 and the second lamp is of
the type described in connection with FIG. 1.
If both lamps are operated at about 600 watts per inch the conveyor
may be run at at least about 9 meters per minute. Effective cure is
obtained at about 32 joules of lamp input power per square
centimeter of substrate.
EXAMPLE
The coatings utilized for testing were supplied by Crown Metro,
Inc., located in Greenville, S.C., USA.
Two lamp units each about 10 inches long and spaced about 9 inches
apart were used. The reflectors were half-ellipses in cross-section
with the bulb lying along the focus of the cross-section. The
opening of the reflector was spaced about 1 inch above the upper
surface of a one inch thick substrate which positioned the second
focus at about the under surface of the substrate.
Two coatings were evaluated, a high gloss black UV topcoat #EA090W
and a high gloss white UV topcoat #EA074W. Each of the coatings
were applied to MDF.
The optimum process conditions were established as follows:
Black UV High Gloss Top Coat
(1) Spray apply 3 to 7 mils of coating to a 1 inch thick
substrate
(2) Flash-off solvent for 2 minutes at 120.degree. F.
(3) Cure in one pass with the "D" lamp operating at 160 w/cm (input
power) followed by non-doped bulb at 240 w/cm.(input power)
(4) Rate of cure 9 meters/min.
White High Gloss Topcoat
a) Spray apply 3 to 5 mils of coating
b) Flash-off solvent for 2 minutes at 120.degree. F.
c) Cure in line with the "V" bulb followed by a non-doped bulb,
each operating at 240 w/cm(input power)
d) Rate of cure 9 meters/min.
The pencil hardness of both coatings measured according to ASTM
test standard 0-3363-74 was greater than 6H.
The gloss at 60 degrees for both coatings measured by a Gardner
gloss meter was greater than 90 gloss units.
The lamp input power is directly related to the ultraviolet output.
The above example of curing a white coating requires both lamps to
operate at 240 watts/cm for the white coating as opposed to one at
160 watts/cm for the black coating. This difference may be due to
reflectivity of the white coating.
For the white coating process described above, the lamp input
energy per square centimeter of cured coating is 32 joules. For the
black coating process, it is 262/3 joules.
Conveyor speed of up to 9 meters per minute have been obtained for
two lamp curing systems according to the invention. It is
contemplated that increasing the number of lamps would allow a
proportional increase in the conveyor speed.
Although examples have been given for only black and white
coatings, the present invention is applicable to other coating
colors. In the Munsell system of indicating color by three
coordinates, the value number indicates the relative darkness and
lightness of the color. In accordance with the Munsell system, the
word "dark" herein includes a color that has a Munsell number of
less than 5 and "light" includes a color that has a Munsell number
of 5 or more.
* * * * *